The study, which was performed by Emi Nishimura, a professor at Kanazawa University, and colleagues on mice, was published in the journal Cell. It found that genotoxic stress (anything that injures a cell’s genetic code) affects so-called melanocyte stem cells, the unspecialized cells that supply, through cell division, the cells (melanocytes) that provide the pigment for hair coloration. When the stem cells’ DNA is damaged, they go haywire, and themselves change (differentiate) into mature melanocytes and divert themselves into a place in the hair follicle where they aren’t able to color the hair.

Even though their work was done on mice, the scientists said they believe their conclusions are transferable to humans. In fact, they speculated their results might explain why patients with ataxia telangiectasia, a rare, neurodegenerative disorder caused by a flaw in a gene called ATM, get prematurely gray hair. The ATM gene is like a “caretaker” that ordinarily protects melanocyte stem cells. But when the gene is damaged, the stem cells can go awry.

Other scientists, however, believe that grayness is generated by a large buildup of hydrogen peroxide from aging of the hair follicles. The hydrogen peroxide blocks normal production of melanin, the pigment produced by melanocytes, a team of European scientists recently reported in the FASEB Journal, published by the Federation of the American Societies for Experimental Biology.